Wind profilers are radars that are able to detect scattering of their electromagnetic energy by the atmosphere when there are no precipitation particles or hydrometers present. These radars are often referred to as "clear-air" radars to distinguish them from weather radars. The targets that cause the scattering for clear-air radars are the atmospheric inhomogeneities along the path of the transmitted radar beam. The present invention is particularly applicable to these types of radar, but more generally is applicable to any pulsed radar system in which a region of space is examined by transmitting normally rectangular radar pulses thereto and analyzing the resulting scattered and/or reflected energy.
The necessary bandwidth of a pulsed radar transmitter, such as used in a wind profiler, is defined as the frequency range between the first two zeroes in the Fourier spectrum of the transmitted pulse. For a perfect rectangular pulse envelope this is equal to two divided by the pulse duration. Any energy falling outside of this frequency range contributes nothing to the performance of the radar, but can cause problems with interference to other nearby radio services.
The occupied bandwidth of any pulsed radar, is defined as the frequency range containing ninety-nine percent of the transmitted power. This includes the useful power contained within the necessary bandwidth, and the power outside of the necessary bandwidth. The necessary bandwidth and the occupied bandwidth are shown graphically in FIG. 1. It is desirable to increase the proportion of the transmitted power contained within the necessary bandwidth in relation to that of the occupied bandwidth. A common practice is to compare the occupied bandwidth to the necessary bandwidth, expressed as the ratio of the occupied bandwidth to necessary bandwidth. A lower occupied bandwidth to necessary bandwidth ratio is indicative of a more efficient use of the RF spectrum.
Pulsed radars and wind profilers typically operate with transmit pulses with nearly rectangular leading and trailing edges. This rectangular pulse tends to have a rather large occupied bandwidth to necessary bandwidth ratio. One way to reduce the ratio of occupied to necessary bandwidth is by shaping the transmit pulse into something other than a rectangular pulse.